less concentrated. The path by which natural gas travels as it dissipates is referred to as its “path of migration.” Because natural gas is normally odorless, the non-toxic chemical mercaptan is added to allow easy detection due to its “rotten egg” smell. Mercaptan provides a powerful first defense in detecting a natural gas leak. The smell can be detected in natural gas concentrations as low as 1%, well below the LFL of 5%. The inadvertent release of natural gas can occur in several ways. For example, a vehicle may have a slow leak due to an improperly sealed fuel system component.
Such minor releases may also occur when disconnecting fuel system components during a repair such as a fuel filter change. In general, it is unlikely that combustible concentrations of natural gas will result from minor leaks that occur over time because the natural gas released from these types of leaks will dissipate rather quickly. The path by which natural gas travels as it dissipates is Though rare, a creditable release of natural gas may occur if a CNG tank or fitting fails resulting in the entire volume of the tank emptying rapidly.
To prevent against creditable releases, CNG tanks are equipped with a pressure relief device that is set to open in the event of excessive pressure or temperature, releasing smaller and more controlled volumes of natural gas and allowing the vehicle system and tank pressure to return to a safe range. referred to as its path of migration. The combustible range for natural gas is from 5% to 15% by volume. Gas concentrations at the center of a release are typically too rich (above the 15% concentration) to support combustion. However, as the gas disperses, the concentration diminishes to a fuel/air ratio that is capable of combustion.
As dispersion continues, the concentra tion decreases to where it becomes too lean (below 5% concentration) and is no longer combustible. During the initial release, it is not uncommon to have natural gas concentrations within the combustible range until the concentration diminishes over time. A facility must be modified to eliminate potential ignition sources that may come into contact with released gas, as well as to ensure that the air circulation exhausts gas as quickly as possible.
Unlike gasoline or diesel vapors, the ignition of natural While a CNG maintenance facility is designed to safely handle all types of releases, it is the “creditable” uncon trolled release that presents the greatest danger. Although this type of event is extremely rare, a maintenance facility must be designed to protect against it. gas can only occur when concentration levels are within the 5%–15% range. Natural gas can be ignited by a spark, flame, or very hot surface. To eliminate the hazard of ignition, a mainte nance facility must be modified to eliminate potential ignition sources that may come into contact with released gas.
Additionally, a maintenance facility must be modified to ensure that the air circulation exhausts gas as quickly as possible to minimize the amount of time that gas con centrations are within the combustible range. Because natural gas is lighter than air, released gas is likely to experience interruptive forces as it disperses throughout the structure. Paths of migration are also affected by air movement within the structure. For this reason, a maintenance facility must be designed to prevent released gas from entering unprotected areas as it rises and disperses. This is accomplished by installing Figure 1.
Unsealed conduit or pipes passing through walls or ceilings present a potential path of migration. Photos courtesy of Gladstein, Neandross & Associates, NREL 44893, 44894, and 44895 SEPTEMBER 2017 9